docs: def make_virtualenv(env):
      """ Create a virtualenv """
      if find_executable("virtualenv") is not None:
          cmd = ["virtualenv"] + [env]
          subprocess.check_call(cmd)
      else:
          # Otherwise, download virtualenv from pypi
          path = urlretrieve(VENV_URL)[0]
          subprocess.check_call(["tar", "xzf", path])
          subprocess.check_call(
              [sys.executable, "virtualenv-%s/virtualenv.py" % VENV_VERSION, env]
          )
          os.unlink(path)
          shutil.rmtree("virtualenv-%s" % VENV_VERSION)